Mixing process for combustion furnaces

ABSTRACT

A system and method for increasing combustion furnace efficiency, including the steps of providing a furnace with a plurality of secondary air injection ducts, asymmetrically positioned in an tangentially reinforcing manner; injecting fuel with substoichiometric primary air through the burners; injecting secondary air through the plurality of secondary air injection ducts; wherein the velocity of the injected air is such that the ratio of the advected air velocity to the furnace width is between about 2 sec −1  to about 150 sec −1 ; thereby increasing combustion efficiency and reactor efficiency via mixing and rotation of the combustion space.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates generally to a system and method for improving the efficiency of combustion reactions, and, more particularly, to a system and method for improving the efficiency of furnaces.

[0003] (2) Description of the Prior Art

[0004] Increases in fuel costs have required power generation plants seek increases in furnace efficiencies in order to reduce power generation costs. However, NOx formation must also be prevented to comply with environmental regulations. NOx formation is reduced in furnaces by the process of stage combustion, which includes administering an initial substoichiometric or suboptimal ratio of oxygen to fuel to maintain combustion gas temperatures below the peak NOx-producing temperature, about 2,800 degrees F. (approximately 1540 degrees C.), followed by the addition of secondary air, or over-fire-air (OFA), to finish the combustion reaction. Proper mixing of secondary air and combustion gases inside a furnace is thus important to achieve optimum combustion and has been improved by the use of rotating over-fire-air (ROFA). However, these existing NOx reduction systems do not optimize combustion efficiency or furnace heat exchange efficiency.

[0005] Therefore, a need exists to improve furnace energy efficiency of ROFA systems without affecting NOx reduction.

SUMMARY

[0006] The present invention is directed to a mixing process and system for increased combustion efficiency.

[0007] The present invention is further directed to a system and method for increased furnace efficiency through increased retention time in the furnace. In a preferred embodiment, the process employs systems and methods to improve the reaction homogeneity and combustion zone swirling, resulting in combustion efficiency gains and thermal flux gains with corresponding gains in reactor efficiency.

[0008] The present invention is directed toward increasing furnace energy efficiency via increased combustion efficiency and increased furnace thermal flux.

[0009] It is one aspect of the present invention to increase combustion efficiency by the induction of turbulence in the gas column. Another aspect of the present invention is to increase thermal flux in a furnace by increasing the residence time of combustion gases in furnace and decreasing the laminar flow at heat exchange surface. In the present invention, these parameters are increased by the induction of turbulence in the combustion gases and at the combustion gas/furnace interface.

[0010] Furthermore, the present invention increases the combustion efficiency through the rapid, thorough mixing of the injected secondary air with the combustion gases via increased turbulence. This rapid, thorough mixing effects a more complete burning of the fuel while reducing the secondary air requirements.

[0011] These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a side view of a combustion furnace operated according to the present invention.

[0013]FIG. 2 is a cross-sectional view of Zone A of the furnace of FIG. 1 showing the gas swirl and deflection turbulence induced by operation according to the present invention.

[0014]FIG. 3 is a cross-sectional view of Zone A of the furnace of FIG. 1 showing the gas rotation induced by operation according to the present invention.

[0015]FIG. 4 is a cross-sectional view of Zone B of the furnace showing the turbulence induced by rotation in a non-circular furnace.

[0016]FIG. 5 is a cross-sectional view of Zone C of the furnace showing the swirl, deflection, and rotation-induced turbulence induced by operation according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as “forward,” “rearward,” “front,” “back,” “right,” “left,” “upwardly,” “downwardly,” and the like are words of convenience and are not to be construed as limiting terms.

[0018] Referring now to the drawings in general, the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. Shown in FIG. 1 is a side view of a combustion furnace, generally described as 12, equipped with an air injection system composed of injection ports 14. As best seen in FIGS. 2 and 3, the present invention provides for an air injection system that creates swirl 20, peripheral turbulence 24, and air column rotation 30 through the tangential injection of secondary air into the furnace. The present invention thus creates turbulence and improves mixing of the overfire air with the combustion gases.

[0019] According to the present invention, the overfire air is injected into the combustion gases at a velocity and orientation such that the swirl and high turbulence generated in the combustion gases achieve a rapid and thorough mixing of the advected gases and the combustion gases.

[0020] As shown in FIG. 2, injection of the overfire air into the combustion gases is effected in a manner such that the advected air travels across the column of combustion gases and is deflected by the opposing wall. This forceful injection induces turbulent mixing of the advected air and combustion gases in at least three ways: 1) by the generation of swirl 20 in the gas column, 2) the generation of turbulence in proximity of the opposing wall after deflection of the advected air by the wall 24, and 3) by the turbulence caused by the rotation of the column of combustion gases in a non-circular furnace, shown as 26 in FIG. 4. Swirl 20 is also generated by the rotation of the gas column, as shown in FIG. 4.

[0021] The rotation, shown as 30 in FIG. 3, is produced through the tangential injection into the furnace of the advected ROFA air, i.e. there is an injection port on each side of the furnace. The injection port on the right may be, for example, toward the rear of the furnace while the injection port on the left side may be toward the front side of the furnace. This placement of ports results in a “swirl” being created in the furnace much like the injection of water in a whirlpool can create a swirl, resulting in mixing, such as described in U.S. Pat. No. 5,809,910 issued Sep. 22, 1998 to Svendssen. This system provides for the asymmetrical injection of overfire air (OFA) in order to create turbulence in the furnace, thus more thoroughly mixing the secondary air and the combustion gases.

[0022] Turbulence generated in proximity of the opposing wall is achieved when the advected air strikes the opposing wall before being completely mixed into the combustion gases. That is, the penetration of the injected secondary air is greater than the width of the furnace and the secondary air deflects off the opposing wall and generates turbulent flow. To achieve penetration and, therefore, turbulence, the advected gas must have sufficient linear momentum to penetrate the primary gas, strike the deflecting surface, and rotate. This linear momentum is described as mass flow for a continuous gas stream. The mass flow (m) of a fluid is defined as follows:

m=density of fluid×Area×average fluid velocity normal to Area

[0023] The mass flow of the advected gas must be sufficient to traverse the column of flue gas, strike the deflecting surface, and create turbulence. The distance from injection to deflection, represented by the width of the flue gas chamber, dictates the necessary mass flow required to achieve turbulence. However, since the desired rate of added gas mass is limited, it is often desirable to increase the velocity of the advected gas, thereby increasing the mass flow. Thus, greater mass flow of the advected air can be attained by increasing the velocity of the gas.

[0024] Rotation of combustion gas column in a furnace with a non-circular cross-section causes additional turbulence formation due to the non-circular cross-section. The rotation is achieved, as previously described, by the use of opposing, coordinated, tangential injection of secondary air into the combustion gas column. Thus, rotation of the gas column in a non-circular cross-section furnace produces rotation-induced turbulence, especially at the furnace/gas interface.

[0025] In a system according to the present invention, a series of gas ducts with nozzles advecting gas into a moving column of flue gas are positioned in a predetermined manner to create rotational flow of the combustion zone, as described in U.S. Pat. No. 5,809,910, incorporated herein by reference in its entirety. The injection ducts are preferably arranged to act at mutually separate levels on the mutually opposing walls of the furnace, as shown in FIGS. 1 and 2 of an incineration unit and/or are displaced laterally in pairs in relation to one another. Additionally, the nozzles are preferably positioned at successively increasing distances along the axis of flow of the furnace away from the furnace, as shown in FIG. 1, such that rotation is maintained by the coordinated, reinforcing, tangential injection of high-velocity secondary air into the combustion gas column, generally described as 50 in FIG. 5.

[0026] A fourth means of producing turbulence is through the advection of overfire air or gases that are cooler than the combustion gases. This cooler air produces additional turbulence from the thermal expansion it undergoes upon mixing with the combustion gases. That is, the advected gas expands as it is warmed to the combustion gas temperature by the combustion gas, thus displacing and further mixing the surrounding combustion gas. However, in the case of combustion power plants, the advected air should not be so cold as to reduce the temperature of the exiting combustion gases and thus reduce heat exchange efficiency. In these furnaces, ambient air between −20 and 100 degrees centigrade (−4 to 212 degrees F.) can be used in the advected gas. Preheated gas, such as from redirected combustion air, may also be used in the advected gas. The redirected combustion air is preferably between 100 and 500 degrees centigrade (200 and 930 degrees F.) and is preferably mixed, if needed, with the ambient air at between 10 to 50% of the total advected gas, to provide an advection gas with temperature of between about 40 and 460 degrees centigrade. More preferably, the redirected combustion air is mixed at 20-40% of the total advected gas, if needed to provide an advection gas with temperature of between about 76 and 340 degrees centigrade. This gas mixture is therefore warm enough not to reduce the combustion gas temperature significantly and can also readily participate in the combustion reaction upon mixing with the combustion gas.

[0027] These turbulences can thus be further augmented by using high-velocity secondary air. During testing of the system, secondary air was injected into furnaces of various sizes at velocities ranging from 60-300 m/s using booster fans. The velocity necessary to provide sufficient mixing is dependent upon the size of the furnace, the vertical velocity of the combustion gasses and the configuration of the furnace.

[0028] Surprisingly, the turbulence generated was sufficient that the entire furnace began operating as a single burner. The increased turbulence, mixing swirl, and rotation in the furnace resulted in improved combustion, increased efficiency of the fuel combustion, reduction in secondary air requirements with consequential increased retention time of the combustion gases in the furnace, lower furnace exit gas temperatures due to better heat exchange in the furnace, increased boiler efficiency and lower pollutant emissions.

[0029] From the tests it was determined that the ratio of the advected air velocity to the furnace width (v/w) needs to be between about 2 to about 150 sec⁻¹, preferably between about 3 and 60 sec⁻¹.

[0030] Furthermore, it was determined that the velocity of the advected air should result in the combustion gas column rotating at least one half-turn prior to exiting the furnace, more preferably at least 1 turn prior to exiting the furnace. To achieve this rotation, at least two levels of injection are required. More preferably at least three levels of injection are used.

[0031] Alternatively, the velocity of the injected air needs to be such that the penetration of the injected air is greater than the furnace width by at least about 1.5 furnace widths, more preferably by at least 2 furnace widths.

[0032] The reduction in the secondary air results in a decrease in combustion gas volume, which results in an increased residence time of the combustion gases in the furnace and thus more time for thermal flux to occur into the furnace water/steam conduits.

[0033] Additionally, the rotation of furnace gas column in a non-circular furnace generates turbulence at the gas/furnace surface interface. This turbulence reduces the laminar flow of the combustion gases at the interface and therefore improves the heat transfer across the interface. The turbulence generated by the rotation also further mixes the combustion gases and reduces laminar or parallel flow up the furnace. Combustion reactions in prior art non-circular furnaces tend to demonstrate sidedness, that is the reactions are on a particular side or zone of the furnace versus other sides, resulting in non-uniform combustion. Thus, the present invention advantageously utilizes the non-circular nature of the furnace to eliminate the sidedness of the furnace. The rotation that overcomes this sidedness is achieved by the co-ordinated, reinforcing, tangential injection of high-velocity secondary air into the combustion gas column.

[0034] Similarly, the vigorous mixing in the combustion area produced by the present invention also prevents the laminar flow and consequential lower residence time of higher inertia particles in the furnace, such as combustible particulate, thereby allowing them more time to burn in the furnace and further increasing the combustion efficiency and thermal flux efficiency of the furnace.

[0035] Thus, the present invention utilizes the co-ordinated, reinforcing, tangential injection of high-velocity secondary air to improve the combustion efficiency and thermal flux efficiency of furnaces.

[0036] A method according to the present invention for increasing combustion furnace efficiency includes providing a furnace with a plurality of secondary air injection ducts, asymmetrically positioned in an opposing manner; injecting fuel with primary air through the burners prior to the injection of secondary air; injecting secondary air through the plurality of secondary air injection ducts at a velocity such that the ratio of the velocity to the furnace width is between about 2 sec⁻¹ to about 150 sec⁻¹, preferably between about 3 and about 60 sec⁻¹; thereby increasing combustion efficiency and reactor efficiency via mixing and rotation of the combustion space.

[0037] Alternatively or additionally, the velocity of the injected air is such that the penetration of the injected air is greater than the furnace width by at least about 1.5 widths and/or the combustion zone rotates at least one half revolution prior to exiting the furnace.

[0038] Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. All modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims. 

We claim:
 1. A method for increasing combustion furnace efficiency, comprising: providing a furnace with a plurality of secondary air injection ducts, asymmetrically positioned in an opposing manner; injecting fuel with primary air through the burners prior to injection of secondary air; injecting secondary air through the plurality of secondary air injection ducts; wherein the velocity of the injected air is such that the ratio of the velocity to the furnace width is between about 2 sec⁻¹ to about 150 sec⁻¹; thereby increasing combustion efficiency and furnace efficiency via swirl, peripheral turbulence, and rotation-induce turbulence of the combustion space.
 2. The method of claim 1, wherein the temperature of the injected air is between about 40 and about 460 degrees centigrade (10-50% redirected combustion air).
 3. The method of claim 2, wherein the temperature of the injected air is between about 76 and about 340 degrees centigrade (20-40% redirected combustion air).
 4. The method of claim 1, wherein the system has at least two levels of secondary air ducts for injection of secondary air.
 5. The method of claim 4, wherein the system has at least three levels of secondary air ducts for injection of secondary air.
 6. The method of claim 1, wherein the velocity of the injected air is such that the ratio of the velocity to the furnace width is between about 3 sec⁻¹ to about 60 sec⁻¹.
 7. A method for increasing combustion furnace efficiency, comprising: providing a furnace with a plurality of secondary air injection ducts, asymmetrically positioned in an opposing manner; injecting fuel with primary air through the burners prior to injection secondary air; injecting secondary air through the plurality of secondary air injection ducts; wherein the velocity of the injected air is such that the penetration of the injected air is greater than the furnace width by at least about 1.5 widths; thereby increasing furnace efficiency via mixing and rotation of the combustion space.
 8. The method of claim 7, wherein the temperature of the injected air is between about 40 and about 460 degrees centigrade (10-50% redirected combustion air).
 9. The method of claim 8, wherein the temperature of the injected air is between about 76 and about 340 degrees centigrade (20-40% redirected combustion air).
 10. The method of claim 7, wherein the system has at least two levels of secondary air ducts for injection of secondary air.
 11. The method of claim 10, wherein the system has at least three levels of secondary air ducts for injection of secondary air.
 12. A method for increasing combustion furnace efficiency, comprising: providing a furnace with a plurality of secondary air injection ducts, asymmetrically positioned in an opposing manner; injecting fuel with primary air through the burners prior to injection secondary air; injecting secondary air through the plurality of secondary air injection ducts; wherein the velocity of the injected air is such that the combustion zone rotates at least one half revolution prior to exiting the furnace; thereby increasing furnace efficiency via mixing and rotation of the combustion space.
 13. The method of claim 12, wherein the temperature of the injected air is between about 40 and about 460 degrees centigrade (10-50% redirected combustion air).
 14. The method of claim 13, wherein the temperature of the injected air is between about 76 and about 340 degrees centigrade (20-40% redirected combustion air).
 15. The method of claim 12, wherein the system has at least two levels of secondary air ducts for injection of secondary air.
 16. The method of claim 15, wherein the system has at least three levels of secondary air ducts for injection of secondary air. 